Geckos have specially adapted footpads on their feet, with each square millimeter of footpad containing about 14,000 foot hairs or setae. This provides them with remarkable adhesive capability, allowing them to adhere to most surfaces as a result of van der Waals interactions between the setae and a surface. Each seta has a diameter of about 5 micrometers and is tipped with between 100 and 1,000 terminal bristles or spatulae, with each spatula having a length of about 0.2 micrometers. Each foot hair thus has a hierarchical structure comprising a seta with multiple spatulae. Rigidity from the relatively thick setae fibrils and flexibility from the relatively fine spatulae fibrils provide excellent conformal contact between footpad and surface such that a gecko can support about eight times its own weight with just one toe on smooth glass.
As the adhesive capability of gecko footpads is achieved without requiring liquid or surface tension, it is desirable to develop a man-made surface mimicking the setae structure on gecko footpads having similar adhesive properties arising from the surface structure alone, so as to obtain a completely dry adhesive that does not leave any residue and can be reused. This will be especially useful in low load applications such as pick-and-place handling of delicate microchips, so as to avoid load deformation or damage due to contact with chemical adhesives.
Template-assisted fabrication, based on deposition or introduction of a desired material into arrays of micro- and nano-channels, is a method of forming one-dimensional micro- and nanostructures. Considerable efforts have been made in designing and controlling fabrication of templates for forming gecko-like micro/nanostructures, including electron beam lithography, intermediate film moulding or photolithographic methods followed by silicon micromachining to construct templates for casting or hot embossing polymeric structures.
Among potential templates, nano-porous anodic alumina (NPAA) has drawn attention because it provides a self-assembled array of relatively uniformly-sized parallel pores or channels with large depths, in large scale and low cost. Availability of NPAA films has triggered investigations into their utilization as templates or masks to fabricate various nanostructures including nanotubues, nanowires, nanoporous films and nanodot arrays for wide ranging applications such as catalysis, electronics, optics, and biosensing. It has been reported that low viscosity polymeric solutions were used to cast polymer in NPAA templates in attempts to fabricate nanofibrils. However, long solvent evaporation times (e.g. 24 hours) make this an unsuitable method for use in mass production. Furthermore, the long and thin nanotube fibrils are apt to bend, entangle or clump together especially during removal of the NPAA template by wet etching, thereby impairing adhesive efficacy. Attempts have also been made to produce hierarchical PMMA fibrils using bonded alumina membranes or intermediate film moulds. However, these methods are complex and time consuming as they require precise alignment of multiple layers of membranes or moulds and involve numerous fabrication steps.